In the field of data communications, a modem is used to convey information from one location to another. Digital subscriber line (DSL) technology now enables modems or other devices to communicate rapidly large amounts of data. Devices communicate by modulating a baseband signal carrying digital data, converting the modulated digital data signal to an analog signal, and transmitting the analog signal over a conventional copper wire pair using techniques that are well known in the art. These known techniques include mapping the information to be transmitted into a signal space constellation, encoding the information to reduce errors and improve throughput, and transmitting the information over a communication channel. The constellation can include digital information, or analog and digital information. At a receiver, the signal is received, equalized, and decoded in accordance with techniques that are known in the art.
In the above mentioned communications system, a control device, i.e., one that is located at a telephone company central office, connects the communication channel to a plurality of remote devices typically located at a customer residential or business location. This topography is known as a multipoint communication environment because one control device is communicating with a plurality of remote devices co-located at a remote location, over a single communication channel.
The communication technique between the control device and the remote device is generally half duplex in nature, meaning that only one device may transmit at any particular time. Existing techniques such as carrierless amplitude/phase modulation (CAP) and discrete multitone (DMT) modulation allow modems to transmit simultaneously between only two devices at a time. In a multipoint environment, greater circuit efficiency is possible because of the ability to connect multiple devices to the same communication channel.
In any communication environment, noise is an ever-present obstacle to optimal receiver performance. Noise imparted by the communication channel can be substantially eliminated through the use of well-known techniques, such as preceding and channel equalization. Local noise imparted to a channel, such as periodic impulse noise, from local sources such as electrical appliances and light dimmers, and random impulse noise, such as switching relays in a central office (CO), present an even greater problem that can degrade receiver performance.
DSL equipment is susceptible to these impairments. In order to combat crosstalk, carrierless amplitude/phase (CAP) modulation uses a precoder, while discrete multi-tone (DMT) selectively disables the affected frequency bins. The 60 Hz periodic impulse noise (for example, that generated by a local electrical appliance, such as a light dimmer) and other impulse noise generated, for example, by a switching relay at the central office is allowed to exist. However, the resulting errors are corrected by a forward error correction code, sometimes used in combination with a bit-wise interleaver.
This arrangement works fairly well in point-to-point applications where the delay caused by the coding is not problematic, however, for multipoint applications, the codes are ineffective. For example, the precoder used with CAP is located in the transmitter, and will cause discontinuities when the transmitter is switched on and off, as frequently occurs in a multipoint environment. Furthermore, the control device in the multipoint environment can have but one set of precoder coefficients for broadcasting to all remote devices, which may have different precoder needs. In addition, ringing caused by on/off polling cannot be canceled by a precoder because the transmitter and precoder are inoperable during the ringing transient.
Other error correction codes are available which can be used to reduce some of the errors caused by impulse noise. For example, block codes, and more specifically, linear block codes have been developed for use in communications systems to correct or reduce the number of burst errors in data transmission. However, these codes have excess delay and have been unable to successfully reduce or eliminate the type of random impulse noise generated by relay switching equipment in a telephone company central office.
Therefore, it would be desirable to provide a noise suppression system and method in both a transmitter and a receiver to reduce or eliminate the impulse noise imparted to a receiver, without the need for forward error correction.